1. Field of the Invention
The present invention relates to the selective hydrogenation of diolefins and acetylenic compounds in a olefin rich stream. More particularly the invention relates to a process utilizing a hydrogenation catalyst in a structure to serve as both the catalyst and as a distillation structure for the simultaneous reaction and separation of the reactants and reaction products.
2. Related Art
Mixed refinery streams often contain a broad spectrum of olefinic compounds. This is especially true of products from either catalytic cracking or thermal cracking processes. These unsaturated compounds comprise ethylene, acetylene, propylene, propadiene, methyl acetylene, butenes, butadiene, amylenes, hexenes etc. Many of these compounds are valuable, especially as feed stocks for chemical products. Ethylene, especially is recovered. Additionally, propylene and the butenes are valuable. However, the olefins having more than one double bond and the acetylenic compounds (having a triple bond) have lesser uses and are detrimental to many of the chemical process in which the single double bond compounds are used, for example polymerization. Over the range of hydrocarbons under consideration, the removal of highly unsaturated compounds is of value as a feed pretreatment, since these compounds have frequently been found to be detrimental in most processing, storage and use of the streams.
The C4 cuts are sources of alkanes and alkenes for paraffin alkylation to produce C8 gasoline blending components and as feeds for ether production.
The C5 refinery cut is valuable as a gasoline blending stock or as source of isoamylene to form an ether by reaction with lower alcohols. Tertiary amyl methyl ether (TAME) is rapidly becoming valuable to refiners as a result of the recently passed Clean Air Act which sets some new limits on gasoline composition. Some of these requirements are (1) to include a certain amount of xe2x80x9coxygenatesxe2x80x9d, such as methyl tertiary butyl ether (MTBE), TAME or ethanol, (2) to reduce the amount of olefins in gasoline, and (3) to reduce the vapor pressure (volatility).
The C5""s in the feed to a TAME unit are contained in a single xe2x80x9clight naphthaxe2x80x9d cut which contains everything from C5""s through C8""s and higher. This mixture can easily contain 150 to 200 components and thus identification and separation of the products is difficult. Several of the minor components (diolefins) in the feed will react slowly with oxygen during storage to produce xe2x80x9cgumxe2x80x9d and other undesirable materials. However, these components also react very rapidly in the TAME process to form a yellow, foul smelling gummy material. Thus it is seen to be desirable to remove these components whether the xe2x80x9clight naphthaxe2x80x9d cut is to be used only for gasoline blending by itself or as feed to a TAME process.
The use of a solid particulate catalyst as part of a distillation structure in a combination distillation column reactor for various reactions is described in U.S. patent Nos.: (etherification) U.S. Pat. Nos. 4,232,177; 4,307,254; 4,336,407; 4,504,687; 4,918,243; and 4,978,807; (dimerization) U.S. Pat. No. 4,242,530; (hydration) U.S. Pat. No. 4,982,022; (dissociation) U.S. Pat. No. 4,447,668; and (aromatic alkylation) U.S. Pat. Nos. 4,950,834 and 5,019,669. Additionally U.S. Pat. Nos. 4,302,356 and 4,443,559 disclose catalyst structures which are useful as distillation structures.
Hydrogenation is the reaction of hydrogen with a carbon-carbon multiple bond to xe2x80x9csaturatexe2x80x9d the compound. This reaction has long been known and is usually done at super atmospheric pressures and moderate temperatures using a large excess of hydrogen over a metal catalyst. Among the metals known to catalyze the hydrogenation reaction are platinum, rhenium, cobalt, molybdenum, nickel, tungsten and palladium. Generally, commercial forms of catalyst use supported oxides of these metals. The oxide is reduced to the active form either prior to use with a reducing agent or during use by the hydrogen in the feed. These metals also catalyze other reactions, most notably dehydrogenation at elevated temperatures. Additionally they can promote the reaction of olefinic compounds with themselves or other olefins to produce dimers or oligomers as residence time is increased.
Selective hydrogenation of hydrocarbon compounds has been known for quite some time. Peterson, et al in xe2x80x9cThe Selective Hydrogenation of Pyrolysis Gasolinexe2x80x9d presented to the Petroleum Division of the American Chemical Society in September of 1962, discusses the selective hydrogenation of C4 and higher diolefins. Boitiaux, et al in xe2x80x9cNewest Hydrogenation Catalystxe2x80x9d, Hydrocarbon Processing, March 1985, presents a general, non enabling overview of various uses of hydrogenation catalysts, including selective hydrogenation of a propylene rich stream and other cuts. Conventional liquid phase hydrogenations as presently practiced required high hydrogen partial pressures, usually in excess of 200 psi and more frequently in a range of up to 400 psi or more. In a liquid phase hydrogenation the hydrogen partial pressure is essentially the system pressure.
U.S. Pat. No. 2,717,202 to Bailey discloses a countercurrent process for the hydrogenation of lard carried out in a plurality of independent vertical chamber using a pumped catalyst under undisclosed pressure conditions. U.S. Pat. No. 4,221,653 to Chervenak et al discloses a concurrent hydrogenation for using an ebullating bed at extremely high pressures. UK Patent Specification 835,689 discloses a high pressure, concurrent trickle bed hydrogenation of C2 and C3 fractions to remove acetylenes.
U.S. Pat No. 5,087,780 to Arganbright disclosed a process for the hydroisomerization of butenes using an alumina supported palladium oxide catalyst arranged in a structure for use as both the catalyst and distillation in a catalytic distillation reactor. The hydrogenation of dienes was also observed under high hydrogen partial pressure, in excess of 70 psia, but not at around 10 psia.
It is an advantage of the present process that the diolefins (dienes) and acetylenic compounds contained within the hydrocarbon stream contacted with the catalyst are converted to olefins or alkanes with very little if any formation of oligomers or little if any saturation of the mono-olefins.
The present invention comprises feeding a hydrocarbon stream containing highly unsaturated compounds which comprise diolefins and acetylenes along with a hydrogen stream at an effectuating hydrogen partial pressure of at least about 0.1 psia to less than 70 psia, preferably less than 50 psia to a distillation column reactor containing a hydrogenation catalyst which is a component of a distillation structure and selectively hydrogenating a portion of the highly unsaturated compounds. Within the hydrogen partial pressures as defined no more hydrogen than necessary to maintain the catalyst (most likely to reduce the catalyst metal oxide and maintain it in the hydride state) and hydrogenate the highly unsaturated compounds is employed, since the excess hydrogen is usually vented. This preferably is a hydrogen partial pressure in the range of about 0.1 to 10 psia and even more preferably no more than 7 psia. Optimal results have been obtained in the range between 0.5 and 5 psig hydrogen partial pressure.
The hydrocarbon stream typically comprises C2 to C9 aliphatic compounds, which may be narrow cuts or include a range of carbon content. The invention is the discovery that a hydrogenation carried out in a catalytic distillation column requires only a fraction of the hydrogen partial pressure required in the liquid phase processes which are the form of prior commercial operation for this type of stream, but give the same or better result. Thus the capital investment and operating expense for the present hydrogenation are substantially lower than prior commercial operations.
Without limiting the scope of the invention it is proposed that the mechanism that produces the effectiveness of the present process is the condensation of a portion of the vapors in the reaction system, which occludes sufficient hydrogen in the condensed liquid to obtain the requisite intimate contact between the hydrogen and the highly unsaturated compounds in the presence of the catalyst to result in their hydrogenation.
The highly unsaturated compounds may be present in very minor amounts, i.e., a few parts per million up to major amounts, i.e., over 90 weight %. The present invention may be used to remove impurities or to convert commodity amounts of the highly unsaturated compounds into monoolefins or alkanes as desired.
The hydrogen rate must be adjusted at the partial pressure described such that it is sufficient to support the hydrogenation reaction and replace hydrogen lost from the catalyst but kept below that producing hydrogenation of monoolefins which is understood to be the xe2x80x9ceffectuating hydrogen partial pressurexe2x80x9d as that term is used herein.
As can be readily appreciated the amount of the highly unsaturated compound in the hydrocarbon stream is a factor to be considered in selecting the optimum hydrogen partial pressure, since at least a stoichiometric amount of hydrogen must be present in the system to be available for the reaction. When the highly unsaturated compounds are impurities, present in parts per million the lower range of hydrogen partial pressure is a an excess, but it is necessary because of the scarcity of the selective reactant. Also the nature of this reaction between a gas and a liquid and the apparent need to occlude the hydrogen into the liquid makes an excess of hydrogen within the partial pressures a preferred mode of operation.
An additional feature of the process is that a portion of the mono-olefins contained within the stream or produced by the selective hydrogenation of the diolefins may be isomerized to more desirable products. Isomerization can be achieved with the same family of catalysts as used in hydrogenations. Generally the relative rates of reaction for various compounds are in the order of from faster to slower:
(1) hydrogenation of diolefins
(2) isomerization of the mono-olefins
(3) hydrogenation of the mono-olefins.
It has been shown generally that in a stream containing diolefins, the diolefins will be hydrogenated before isomerization occurs. It has also been found that very low total pressures may be used for optimal results in some of the present hydrogenations, preferably in the range of 50 to 150 psig with the same excellent results. Both higher and lower pressures within the broad range may be used may be used with satisfactory results.